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Pressure manifold design A SWIS consisting of 12 trenches of equal length is to be constructed on a slope. To divide the septic tank effluent equally among the 12 trenches, a pressure manifold is to be used. The lateral taps are to be spaced 6 inches apart on one side of the manifold. Table 4-7 can be used to size the manifold. Looking down the series of columns under the Single -sided manifold, up to sixteen 1/2 -inch taps could be made to a 4 -inch manifold. Therefore, a 4 -inch manifold would be acceptable. If 3/4- or 1 -inch taps were used, a 6 -inch manifold would be necessary. Using the orifice equation, the flow from each lateral tap can be estimated by assuming an operating pressure in the manifold: Q = Ca(2gh)2 where Q is the lateral discharge rate, C is a dimensionless coefficient that varies with the characteristics of the orifice (0.6 for a sharp -edged orifice), a is the area of the orifice, g is the acceleration due to gravity, and h is the operating pressure within the manifold. In English units using a 0.6 orifice coefficient, this equation becomes Q =11.79 dzhdvz where Q is the discharge rate in gallons per minute, d is the orifice diameter in inches, and h is the operating pressure in feet of water. Assuming 1/2 -inch taps with a operating pressure of 3 feet of water, the discharge rate from each outlet is Q= 11.79('/2)23'12=5.1 gpm Thus, the pump must be capable of delivering 12 x 5.1 gpm or approximately 60 gpm against an operating pressure of 3 feet of water plus the static lift and friction losses incurred in the forcemain to the pressure manifold. wastewater from each orifice in the network when fully pressurized. This is accomplished by main- taining a uniform pressure throughout the network during dosing. The manifolds and laterals are sized relative to the selected orifice size and spacing to achieve uniform pressure. A manual flushing mechanism should be included to enable periodic flushing of slimes and other solids that accumulate in the laterals. Figure 4-16. Rigid pipe pressure distribution networks with flushing cleanouts Small Diameter Pressure Distribution Pumping (Dosing) Septic Tank Chamber Cleanout Effluent Pump Design of dosed flow systems A simplified method of network design has been developed (Otis, 1982). Lateral and manifold sizing is determined using a series of graphs and tables ager the designer has selected the desired orifice size and spacing and the distal pressure in the network (typically I to 2 feet of head). These graphs and tables were derived by calculating the change in flow and pressure at each orifice between the distal and proximal ends of the network. The method is meant to result in discharge rates from the first and last orifices that differ by no more than 10 percent in any lateral and 15 percent across the entire network. However, subsequent testing of field installations indicated that the design model overestimates the maximum lateral length by as much as 25 percent (Converse and Otis, 1982). Therefore, if the graphs and tables are used, the maximum lateral length for any given orifice size and spacing should not exceed 80 percent of the maximum design length suggested by the lateral sizing graphs. In lieu of using the graphs and tables, a spreadsheet could be written using the equations presented and adjusting the orifice discharge coefficient. 4-26 USEPA Onsite Wastewater Treatment Systems Manual